Diatomic Input Research Articles

Annual cycle and spatial trends in fatty acid composition of suspended particulate organic matter across the Beaufort Sea shelf

Fatty acid profiles of suspended particulate organic matter (POM) were determined over an annual cycle (September 2003 to August 2004) on the Beaufort Sea shelf, Canadian Arctic. Special emphasis was placed on the nutritional quality of the fatty acid pool available to zooplankton by examining spatial and temporal patterns in the proportions of total polyunsaturated fatty acids (PUFA) and the essential fatty acids 22:6n−3 (DHA) and 20:5n−3 (EPA). EPA and DHA were the two most abundant PUFA throughout the study period. A log-ratio multivariate (LRA) analysis revealed strong structure in fatty acid profiles related to season and depth. Dominant fatty acids accounting for the observed trend included 18:5n−3, 18:4n−3, 16:1n−7, 20:5n-3, 18:0 and 20:3n−3. We observed a shift in fatty acid profiles from summer to autumn (e.g., from 16:1n−7 and EPA to 18:5n−3 and 18:4n−3) that likely corresponded to a shift in the relative importance of diatoms versus dinoflagellates, prymnesiophytes and/or prasinophytes to the POM pool. Fatty acid composition during winter was dominated by more refractory saturated fatty acids. A surprising finding was the depth and seasonal trend of 20:3n-3, which was higher in winter, aligned with 18:0 in the LRA, but behaved differently than other n−3 PUFA. We interpret fatty acid profiles during summer to be predominantly driven by phytoplankton inputs, whereas fatty acid profiles in winter were dominated by fatty acids that were left over after consumption and/or were generated by heterotrophs. The highest diatom inputs (EPA, the diatom fatty acid marker), n−3/n−6 ratios, and C16 PUFA index were located in an upwelling region off Cape Bathurst. This study is the first annual time series of fatty acid profiles of POM in Arctic seas, expanding our knowledge of the composition of POM throughout the dark season.

Trophic niche of two co-occurring ophiuroid species in impacted coastal systems, derived from fatty acid and stable isotope analyses

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 525:127-141 (2015) - DOI: https://doi.org/10.3354/meps11169 Trophic niche of two co-occurring ophiuroid species in impacted coastal systems, derived from fatty acid and stable isotope analyses Aline Blanchet-Aurigny1,*, Stanislas F. Dubois1, Claudie Quéré2, Monique Guillou3, Fabrice Pernet2 1IFREMER, Laboratoire d’Ecologie Benthique, Département Océanographie et Dynamique des Ecosystèmes, Centre de Bretagne, BP70, 29280 Plouzané, France 2IFREMER, Laboratoire de Physiologie Fonctionnelle des Organismes Marins, LEMAR UMR CNRS IRD 6539, Centre de Bretagne, BP70, 29280 Plouzané, France 3Institut Universitaire Européen de la Mer, Université de Bretagne Occidentale, LEMAR UMR CNRS IRD 6539, place Nicolas Copernic, 29280 Plouzané, France *Corresponding author: ablanche@ifremer.fr ABSTRACT: The trophic niches of 2 common co-occurring ophiuroids, Ophiocomina nigra and Ophiothrix fragilis (Echinodermata, Ophiuroidea), in 2 contrasting coastal systems of Brittany (France) were investigated. We used a combination of fatty acid biomarkers derived from neutral lipids and stable isotopic compositions to explore the contributions of oceanic versus continental inputs to the ophiuroids’ diet. We investigated 2 different systems with an inshore versus offshore comparison. We sampled potential food sources and surveyed organisms every 2 mo for 1 yr. Spatio-temporal variations in stable isotopes and fatty acid profiles of the ophiuroids were generally low compared to interspecific differences. Fatty acid markers showed that both ophiuroids relied on diatom inputs. However, a more δ15N-enriched isotopic composition as well as a more balanced plant- versus animal-derived fatty acid composition in O. nigra suggest that a broader range of food sources are being used by this species irrespective of location or sampling time. The positive correlation between the 18:1n-9/18:1n-7 fatty acid ratio and δ15N values indicates a higher trophic position for O. nigra (suggesting an omnivorous feeding mode), whereas O. fragilis appears to be more herbivorous. Moreover, the low polyunsaturated/saturated fatty acid ratio associated with elevated bacterial fatty acid markers indicates that O. nigra preferentially consumes detritus, while O. fragilis relies more on fresh phytoplankton-derived material. Both stable isotope and fatty acid analyses suggest that terrestrial inputs do not contribute significantly to the diet of these ophiuroids. However, phytodetritus derived from decomposing green macroalgae contributed to the diet of O. nigra in the Bay of Douarnenez. KEY WORDS: Echinoderm · Feeding ecology · Ophiocomina nigra · Ophiothrix fragilis · Trophic markers · Green algae Full text in pdf format Supplementary material PreviousNextCite this article as: Blanchet-Aurigny A, Dubois SF, Quéré C, Guillou M, Pernet F (2015) Trophic niche of two co-occurring ophiuroid species in impacted coastal systems, derived from fatty acid and stable isotope analyses. Mar Ecol Prog Ser 525:127-141. https://doi.org/10.3354/meps11169 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 525. Online publication date: April 09, 2015 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2015 Inter-Research.

Vegetation response to southern California drought during the Medieval Climate Anomaly and early Little Ice Age (AD 800–1600)

High-resolution studies of pollen in laminated sediments deposited in Santa Barbara Basin (SBB) core SPR0901-02KC reflect decadal-scale fluctuations in precipitation spanning the interval from AD 800–1600. From AD 800–1090 during the Medieval Climate Anomaly (MCA) SBB sediments were dominated by xeric vegetation types (drought-resistant coastal sagebrush and chaparral) implying reduced precipitation in the southern California region. Drought-adapted vegetation abruptly decreased at AD 1090 and was rapidly replaced by mesic oak (Quercus) woodlands associated with an increased pollen flux into the basin. After a mesic interval lasting ∼100 years, pollen flux and the relative abundance of Quercus pollen dropped abruptly at AD 1200 when the rapid rise of chaparral suggests a significant drought similar to that of the MCA (∼AD 800–1090). This brief resurgence of drought-adapted vegetation between AD 1200–1270 marked the end of the MCA droughts. A gradual increase in mesic vegetation followed, characterizing cool hydroclimates of the Little Ice Age (LIA) in coastal southern California.The presence of xeric vegetation in SBB coincides with major drought events recorded in tree rings and low lake levels elsewhere in California except for the brief drought between AD 1130–1160. Correlative diatom and terrigenous sediment input proxy records from SBB are largely supportive of the pollen record predominantly linking the MCA with drought and La Niña-like conditions and the LIA with wetter (more El Niño-like) conditions. Differences between paleoclimate proxies (pollen, diatoms, and terrigenous sediment) in SBB exist, however, possibly reflecting the temporal and spatial differences in the generation of each proxy record, as well as their individual sensitivity to climate change.

Effect of plankton-derived organic matter on the microbial community of coastal marine sediments

An experimental study was carried out to observe the microbial response to two different plankton-derived organic matter inputs in a coastal sedimentary community of Ubatuba, São Paulo, SE Brazil. The organic enrichment experiment was conducted in order to test experimentally the stimulus of the sediment prokaryotic community after the input of labile material simulating an algal bloom reaching the sea floor. A total of 57 corers (two treatments: the diatom Phaeodactylum tricornutun and the phytoflagelate Tetraselmis sp. and a control) were maintained for a total of 30days in constant temperature, circulation and oxygenation. After the addition of algae an increase in oxygen consumption was observed, accompanied by an increase of prokaryotic total and live density, showing an immediate response from the community to the input of labile material in the sediment. Analyses of molecular fingerprints of bacterial communities by denaturing gel gradient electrophoresis (DGGE) showed differences in bacterial community composition between both treatments and control just after algae addition. This was well evidenced after bacteria genomic libraries analyses that showed differences in diversity and dominance between treatments. In general, Gammaproteobacteria was the most diverse and abundant group in the sediment samples. However, the addition of phytoflagellates led to a shift in dominance in favor of Alphaproteobacteria while diatom input led to a greater bacterial diversity.

Biomarker assessment of spatial and temporal changes in the composition of flocculent material (floc) in the subtropical wetland of the Florida Coastal Everglades

Environmental context Flocculent material (floc) in freshwater and coastal areas of the Florida Everglades plays an important role in food web dynamics and nutrient cycling. Using biomarkers and pigment chemotaxonomy, we determined the organic matter composition of floc from different environments in the Everglades, and found that it is dominated by local biomass inputs and influenced by hydrological regimes. With the on-going restoration of the Florida Everglades, it is important to gain a better understanding of the biogeochemical dynamics of floc, including its sources, transformations and reactivity. Abstract Flocculent material (floc) is an important energy source in wetlands. In the Florida Everglades, floc is present in both freshwater marshes and coastal environments and plays a key role in food webs and nutrient cycling. However, not much is known about its environmental dynamics, in particular its biological sources and bio-reactivity. We analysed floc samples collected from different environments in the Florida Everglades and applied biomarkers and pigment chemotaxonomy to identify spatial and seasonal differences in organic matter sources. An attempt was made to link floc composition with algal and plant productivity. Spatial differences were observed between freshwater marsh and estuarine floc. Freshwater floc receives organic matter inputs from local periphyton mats, as indicated by microbial biomarkers and chlorophyll-a estimates. At the estuarine sites, the floc is dominated by mangrove as well as diatom inputs from the marine end-member. The hydroperiod (duration and depth of inundation) at the freshwater sites influences floc organic matter preservation, where the floc at the short-hydroperiod site is more oxidised likely due to periodic dry-down conditions. Seasonal differences in floc composition were not consistent and the few that were observed are likely linked to the primary productivity of the dominant biomass (periphyton in the freshwater marshes and mangroves in the estuarine zone). Molecular evidence for hydrological transport of floc material from the freshwater marshes to the coastal fringe was also observed. With the on-going restoration of the Florida Everglades, it is important to gain a better understanding of the biogeochemical dynamics of floc, including its sources, transformations and reactivity.

Widespread distribution and molecular diversity of diatom frustule bound aliphatic long chain polyamines (LCPAs) in marine sediments

The siliceous skeletal remains of diatoms are a primary component of sediments, covering vast areas of the sea floor. We used high performance liquid chromatography–electrospray ionization–tandem mass spectrometry (HPLC–ESI–MSn) under high resolution time-of-flight MS (TOF-MS) conditions, as well as nuclear magnetic resonance spectroscopy (NMR) and Fourier transform infrared spectroscopy (FTIR), to investigate the distribution and molecular diversity of long chain polyamines (LCPAs) entrapped in sedimentary diatom biosilica in sediments from the Indian sector of the Southern Ocean, the Bering Sea and the Northeast Pacific. This revealed the existence of complex polyamine populations with both known and unknown structural features, including the presence of ketone and carboxylic acid groups in the LCPA backbone, functional groups that likely modulate the silica precipitating properties of LCPAs in the environment and which may constitute points of initial silica deposition (Wallace, A.F., De Yoreso, J.J., Dove, P.M., 2009. Kinetics of silica nucleation on carboxy- and amine-terminated surfaces: insights for biomineralization. Journal of the American Chemical Society 131, 5244–5250). Although various LCPA species overlapped between samples (putrescine-based LCPAs with various degrees of methylation and N-methyl propylamine repeat units were ubiquitous in all samples), clear regional differences were shown in composition and structural characteristics, likely reflecting the various diatom species contributing to the total LCPA pool for each sedimentary environment. We further show, using scanning electron microscopy and energy dispersive X-ray analysis (SEM–EDX) that LCPAs purified from diatom skeletal remains rapidly re-direct the formation of silica nanospheres in vitro, and become embedded in the precipitated silicified matrix, forming an LCPA–silica composite material. The results show that aliphatic polyamines produced by diatoms for morphogenesis of their cell walls are widespread in sediments where the fossil frustules are preserved. These newly assigned components constitute useful biological markers of diatom input to sediments.

A molecular marker-based assessment of sedimentary organic matter sources and distributions in Florida Bay

Seven surface sediment samples covering the general geographical area of Florida Bay were examined through the measurement of 13C isotopic abundance and lipid classes to assess the distributions and sources of organic matter (OM) in this estuarine environment. The bulk δ13Corg value shifted from a more isotopically depleted (−19.9‰) to a more isotopically enriched (−13.5‰) signal along the NE to SW transect. Two geochemical proxies (Paq and C25/C27n-alkan–2-ones) indicative of seagrass-derived OM significantly increased from near-shore to offshore areas, while taraxerol, a biomarker for mangroves, substantially decreased from 7200 to 284 ng/g along that transect. A clear spatial variation of OM sources was observed in Florida Bay. Generally, the sites in the NE contained mixed OM sources of terrestrial (mangrove) and seagrass-derived OM, where the terrestrial component accounted for over 60% of the OM. In contrast, the sites in central and SW Florida Bay were strongly dominated by seagrass-derived OM. Other lipid fractions such as fatty acids, n-alcohols and sterols revealed an important contribution of algae and bacteria especially in the central and SW section of the Bay. Relatively abundant C25 HBIs suggest important marine diatom inputs, while the presence of C20 HBIs particularly in central Florida Bay possibly reflects the contribution of cyanobacterial mats. The molecular proxies developed in this study to assess OM sources in Florida Bay are promising tools for the characterization and seasonal variability assessment of OM in this and other similar subtropical and tropical estuaries and for paleoenvironmental studies.

Occurrence of C 25 highly branched isoprenoids (HBIs) in Florida Bay: Paleoenvironmental indicators of diatom-derived organic matter inputs

C 25 highly branched isoprenoids (HBIs) are potentially valuable indicators of diatom-derived organic matter (OM) inputs to sediments and the C 25 alkane has been used previously as a biomarker in paleoenvironmental studies. In this work, a suite of C 25 HBI monoenes and dienes was detected in a sediment core from Russell Key, central Florida Bay, USA. Hydrogenation proved all these compounds to be acyclic alkenes with the parent structure of the C 25 HBI alkane 2,6,10,14-tetramethyl-7-(3′-methylpentyl)pentadecane. The tentative double bond positions and geometry of three monoenes and one diene were also established on the basis of comparison of retention indices (RI) and mass spectra with those published for synthetic or isolated compounds. The abundance of individual alkenes showed significantly different depth profiles. The concentrations of dienes decreased rapidly with increasing depth, but this trend was not observed for the monoenes. The highest concentration of total C 25 HBIs was observed at mid depth in the core, suggesting strong historical inputs of diatom-derived sedimentary OM during that period. In fact, the depth profile of C 25 HBIs reflected quite well historical variations in diatom abundance and variations in diatom species composition in central Florida Bay, based on the results of fossil diatom species analysis using microscopy. This study provides further evidence that, with care, some C 25 HBIs can be applied as biomarkers for diatom inputs in paleoenvironmental studies although our data did not allow the identification of one specific diatom group as the main HBI contributor.

Occurrence of novel monocyclic alkenes from diatoms in marine particulate matter and sediments

Widely occurring alkenes reported in sediments from around the globe, but which previously had unknown identities and sources, have now been assigned. They include a novel monocyclic C 25 triene and novel C 30 monocyclic tetra- and pentaenes. Recently, these compounds have been shown to be biosynthesised by some strains of the widespread planktonic diatom, Rhizosolenia setigera, and this appears to be the likely source of the alkenes in the marine environment. The hydrocarbons were identified by comparison of the gas chromatographic retention data and mass spectra of the sedimentary alkenes isolated from the Arabian Sea, Peru upwelling region and Cariaco Trench with data for the pure isolated alkenes from R. setigera. The compounds co-occur with well-known diatomaceous Δ 7(20) highly branched isoprenoid haslenes and rhizenes in all of these environments but were absent from sediments from the Black Sea, even though Δ5 haslenes were abundant in the latter sediments. These differences possibly reflect the different diatom species contributing alkene lipids to each sedimentary environment. The newly identified compounds may be further useful biological markers of diatom inputs to sediments.

Application of 24-norcholestanes for constraining source age of petroleum

Two C 26 sterane ratios, the 24-nordiacholestane ratio (NDR) and the 24-norcholestane ratio (NCR), are useful for assessing petroleum source age. Values for these parameters generally increase in the Cretaceous when diatoms were rapidly radiating and further in the Oligo-Miocene due to widespread high-latitude deposition of diatomaceous sediments. Sources with shale lithologies tend to exhibit the highest ratios; while carbonate and terrigenous-rich lithologies generally show lower ratios. On the North Slope of Alaska, elevated ratios are observed for Cretaceous HRZ oils and mixed oils containing Cretaceous input compared to older Triassic and Jurassic derived oils. Tertiary-sourced oils from Romania and Bulgaria have much higher ratios relative to Jurassic-sourced oils in the Black Sea region. Oleanane was observed in comparable levels in some Black Sea Tertiary and Jurassic oils and source rocks. NDR and NCR coupled with other age-diagnostic indicators (such as oleanane and dinosterane) provide powerful tools for differentiating age of source and organic facies for oils from the Black Sea and Sub-Andean Basins of Peru and Ecuador, respectively. Oils derived from sources deposited in low tropical latitudes tend to have little diatom input and as a consequence, do not display high values regardless of age.

Accurate diatomics-in-molecules calculations on Ar+n clusters

The diatomics-in-molecules approach has been applied to calculate the most stable structures of singly ionized argon clusters (Ar n +, n =3–27). For the first time, highly accurate diatomic input potentials have been employed in such a calculation. As a consequence we obtain by far the most accurate results for the thermal equilibrium structures of Ar n + at 0K to date.

Chlorophyll and carotenoid pigments in solar saltern microbial mats

The distributions of carotenoids, chlorophylls, and their degradation products have been studied in two microbial mat systems developed in the calcite and calcite/gypsum evaporite domains of a solar saltern system. Phormidium valderianum and Microcoleus chthonoplastes are the dominant cyanobacterial species, respectively, and large amounts of Chloroflexus-like bacteria occur in the carbonate/gypsum mat. In both systems, the major pigments are chlorophyll a, zeaxanthin, β-carotene and myxoxanthophyll, which originate from these mat-building cyanobacteria. This common feature contrasts with differences in other pigments that are specific for each mat community. Thus, chlorophyll c and fucoxanthin, reflecting diatom inputs, are only found in the calcite mat, whereas the calcite/gypsum mat contains high concentrations of bacteriochlorophylls c produced by the multicellular green filamentous bacteria. In both cases, the depth concentration profiles (0–30 and 0–40 mm) show a relatively good preservation of the cyanobacterial carotenoids, zeaxanthin, β-carotene, myxoxanthophyll, and echinenone. This contrasts with the extensive biodegradation of cyanobacterial remains observed microscopically. Fucoxanthin in the calcite mat is also transformed at a faster rate than the cyanobacterial carotenoids. Chlorophyll a, the major pigment in both mats, exhibits different transformation pathways. In the calcite/gypsum mat, it is transformed via C-13 2 carbomethoxy defunctionalization prior to loss of the phytyl chain, leading to the formation of pyrophaeophytin a and, subsequently, pyrophaeophorbide a. On the other hand, the occurrence of the enzyme chlorophyllase, attributed to diatoms in the calcite mat, gives rise to extensive phytyl hydrolysis, with the formation of chlorophyllide a, pyrophaeophorbide a and, in minor proportion, phaeophorbide a. Studies of the sources of the photosynthetic pigments and of their transformation pathways in such simplified ecosystems provide a basis for the understanding of the distribution patterns of these compounds in more complex aquatic environments.

The effect of hydrothermal processes on the distribution and stereochemistry of amino acids in Recent Antarctic sediments

Bransfield Strait, Antarctic Peninsula, is a marginal basin located in a heavily sedimented back-arc ridge system that is tectonically and hydrothermally active. Nine sediment samples, ranging in depth from 3 to 7.5 m, were isolated from two gravity cores taken proximal to a volcanic plug in Bransfield Strait. The samples were analyzed for elemental abundances (C, N), stable isotope compositions (δ 13C, δ 15N), and amino acid distributions and stereochemistry in order to assess the effects of hydrothermal stress on organic matter associated with these sediments. In general, %TOC and %N decrease slightly with increasing depth in each core, whereas C/N, % carbonate C and δ 13C and δ 15N remain essentially constant. Total amino acid abundances decrease significantly down core, particularly in the lowermost samples which may have come in direct contact with hydrothermal fluids. Also, with increasing depth, the water extractable “free” amino acids comprise a slightly larger percentage of the total amino acid fraction. Qualitatively, the amino acid distributions reflect the predominance of a diatom input to the sedimentary record. This biogenic signature remains evident despite diagenetic effects that become more pronounced with depth. Individual amino acid D/L values for the total sediment acid hydrolyzates of upper samples in each core remain relatively constant and are consistent with their age. Amino acid D/L values for the lower samples of each core, however, increase dramatically and suggest the occurrence of hydrothermal activity. Inherent complexities of this dynamic system severely limit the possibility of using amino acid racemization data to place constraints on the magnitude and duration of this postulated hydrothermal episode.

Diatomics-in-molecules treatment of NH3

The diatomics-in-molecules method is applied to calculate the ground state potential-energy surface of NH3 by using both the selection of atomic states and the diatomic input data previously employed in a successful DIM description of the low-lying valence states of NH3. It appears that the diatomic information is transferable from one DIM model to the other one only in a qualitative sense. Although the DIM model for NH2 provides a coherent description of the ground state over the region of the configuration space investigated, the model for NH3 is markedly inferior to that of the NH2 radical. Disparities between the DIM results for NH2 and NH3 are discussed.

Assessment of atoms-in-molecules approaches to the diatomic input data generation for the diatomics-in-molecules method

Various versions of the atoms-in-molecules method as a source for diatomic input data, and various approaches of the diatomics-in-molecules method are applied to calculate three lowest valence states of the NH radical. By comparing the results among themselves and with other presently available accurate data, conclusions are drawn concerning the reliability of individual solutions. Also, possible causes for inadequacy of diatomics-in-molecules models are discussed.

LIPIDS IN BLUE‐GREEN ALGAL MATS (MODERN STROMATOLITES) FROM ANTARCTIC OASIS LAKES1

ABSTRACT Lipids comprising the stenols, stanols, polar lipid fatty acids, alkanes and alkenes of blue‐green algal‐(diatomaceous)‐microbial mats and cores (modern cold water stromatolites) collected from three Antarctic lakes were identified and compared with those of other algae. The major stenols were: (cholesta‐5, 22‐dien‐3β‐ol, cholest‐5‐en‐3β‐ol, 24‐methylcholesta‐5, 22‐dien‐3β‐ol, 24‐methyl‐cholest‐5‐en‐3β‐ol, 24‐ethylcholesta‐5, 22‐dien‐3β‐ol, and 24‐ethylcholest‐5‐en‐3β‐ol). The presence of C28 Δ3, 22 stenols, as well as other C28 stenols, was suggestive of diatom input. C29 stenols may have originated from blue‐grern algae. However, the high concentrations of stenols present and the lack of Δ7 stenols was atypical for known stenol components of several blue‐green algal species previously reported. The occurrence of these stenols and other lipid markers strongls implicate diatoms as well as blue‐green algae as important biogenetic sources of lipids and has established the potential for studies of lipid diagenesis in these unique cold, freshwater stromatolites.

Diatomics-in-molecules potential energy surfaces for the collision-induced dissociation of excited by helium atoms

The 1 1 A″, 2 1 A″, 3 1 A′, 4 1 A′, and 5 1 A′ potential energy surfaces (PES) relevant to the collision-induced dissociation (CID) process Li 2 ( 1 Π u ) + He ( 1 S g ) → Li ( 2 S g ) + Li ( 2 P u ) + He ( 1 S g ) are approximated by three diatomics-in-molecules models differing from each other in the source of diatomic input. The semi-empirical model, the most realistic of the three, shows the CID to depend critically on the position of avoided crossings on the PES and on the magnitude of the non-adiabatic coupling, which is estimated numerically. The 1 A″ calculations support a qualitative two-step mechanism recently proposed by Poppe and Whitton. The 1 A′ surfaces exhibit a degree of complexity which precludes a simple picture of the CID process. The results suggest that only a quantum scattering treatment or perhaps a very detailed surface-hopping trajectory calculation will be adequate for the interpretation of the experimental observations on this reaction, and the estimation of the cross sections.

On the diatomic-fragment data calculation for the diatomics-in-molecules method. Applications to NH2 and NH3

Abstract This paper discusses the diatomics-in-molecules theory used in connection with diatomic input data generated by the atoms-in-molecules method and stresses the simplifications associated with the construction of mixed states. The methodical problems are exemplified by treating NH 2 and NH 3 , and conclusions are drawn concerning appropriate DIM models for these species.

The quenching of Na(3 2P) by H2: Interactions and dynamics

An eight-configuration diatomics-in-molecules formalism with symmetrically orthogonalized diabatic input is used to calculate the three lowest potential energy surfaces and nonadiabatic couplings for Na+H2 collisions. The singlet diatomic input for NaH and the singlet and triplet diatomic input for H−2 are chosen to reproduce the accurate adiabatic curves for those cases, and the diatomic input for H2 is based on theoretical interpretations of the resonance states. Trajectory calculations are carried out using the calculated 2 2A′ potential surface and nonadiabatic couplings, and the choice of H−2 input is found to sensitively affect the H2 vibrational excitation in the resonance-line quenching process. One H−2 curve, based on the work of Wadehra and Bardsley, yields an amount of vibrational excitation similar to that observed experimentally. The resulting set of potential energy surfaces provides a reasonable microscopic interpretation of the chemical dynamics of the quenching process Na(3p2P)+H2(v = 0, low j) →Na(3s2S)+H2(v′,j′).

A diatomics-in-molecules case study on the system Be+HF→BeF+H. II. The sensitivity of the potential surfaces to the diatomic input

Potential energy surfaces are presented for a series of diatomics-in-molecules (DIM) models of the reaction Be+HF→BeF+H. This system involves low-lying electronic states of Be and ionic contributions to bonding and is therefore quite complex from the DIM point of view. The various DIM models involve between three and 18 structures and make extensive use of configuration interaction in the diatomic fragments. The potential surface is predicted to have a barrier ∼1.9 eV high for collinear BeFH; at bent configurations this barrier gradually increases. A potential surface having this feature is obtained only if the Be(2s2p) configuration is included in the DIM model. The general problem is raised of how to decide whether a given structure in a DIM model should be retained or not, and of how best to obtain reliable diatomic configuration interaction input data.